Long-Term Debt - Additional Information (Detail) (USD $)	Jun. 30, 2013	May 31, 2013	Jun. 30, 2011 series	Jun. 30, 2010	Jun. 30, 2013 5.68% note, due June 30, 2026 [Member]	Dec. 31, 2012 5.68% note, due June 30, 2026 [Member]	Jun. 30, 2011 5.68% note, due June 30, 2026 [Member]	Jun. 30, 2013 Unsecured senior notes 6.43% [Member]	May 02, 2013 Unsecured senior notes 6.43% [Member]	Dec. 31, 2012 Unsecured senior notes 6.43% [Member]	May 31, 2013 First mortgage bond one [Member]	May 31, 2013 First mortgage bond two [Member]
Debt Instrument [Line Items]
Long term debt maximum borrowing capacity				$ 36,000,000
Total long-term debt	108,600,000				29,000,000	29,000,000	29,000,000	7,000,000	7,000,000	0
Long-term debt, interest percentage					5.70%		5.70%	6.40%	6.40%		9.60%	10.03%
Number of series			2
Amount paid in excess of carrying value deferred as regulatory asset		$ 93,000